The present invention relates to nonaqueous electrolyte secondary cells which comprise a rolled-up electrode unit accommodated in a closed container and serving as an electricity generating element and which are adapted to deliver electric power generated by the electrode unit to the outside.
In recent years, attention has been directed to nonaqueous electrolyte secondary cells, such as lithium ion secondary cells, as cells having a high energy density. The lithium ion secondary cell comprises, as shown in FIGS. 1 and 2, a cylindrical closed container 1 having a cylinder 11 and lids 12, 12 welded to the respective ends of the cylinder, and a rolled-up electrode unit 4 enclosed in the container 1. A pair of positive and negative electrode terminal assemblies 9, 9 are attached to the lids 12, 12, respectively. Each electrode terminal assembly 9 and the rolled-up electrode unit 4 are interconnected by a current collector plate 32 and a lead portion 33 integral therewith, whereby the electric power generated by the electrode unit 4 can be delivered to the outside from the pair of terminal assemblies 9, 9. Each lid 12 is provided with a pressure-relief gas vent valve 13.
As shown in FIG. 3, the rolled-up electrode unit 4 comprises a positive electrode 41 and a negative electrode 43 which are each in the form of a strip and which are lapped over and displaced from each other with a separator 42 interposed therebetween and rolled up into a spiral form. The positive electrode 41 comprises a current collector 45 in the form of aluminum foil and coated with a positive electrode active substance 44. The negative electrode 43 comprises a current collector 47 in the form of copper foil and coated with a negative electrode active substance 46. The active substance 44 of the positive electrode 41 is opposed to the active substance 46 of the negative electrode 43 with the separator 42 interposed therebetween. The positive electrode active substance is a lithium-transition metal composite oxide, while the negative electrode active substance is metallic lithium, alloy for absorbing or desorbing lithium ions or carbon material.
In the charge-discharge reaction of the cell, lithium ions move between the positive electrode active substance 44 and the negative electrode active substance 46 which face each other with an electrolyte positioned therebetween. Stated more specifically, lithium ions migrate from the negative electrode active substance 46 toward the positive electrode active substance 44 and are inserted into the active substance 44 during discharging. During charging, on the other hand, lithium ions are released from the positive electrode active substance 44, migrate toward the negative electrode active substance 46 and are inserted into the active substance 46.
Useful positive electrode active substances are lithium-transition metal composite oxides such as lithium-cobalt composite oxide (LiCoO2), lithium-nickel composite oxide (LiNiO2) and lithium-manganese composite oxide (LiMn2O4). The use of the lithium-transition metal composite oxide as the positive electrode active substance provides a lithium ion secondary cell of four-volt class in discharge voltage and having a high energy density.
Of the lithium-transition metal composite oxides given above, lithium-manganese composite oxide (LiMn2O4) is most favorable with respect to the cost of material and stability of supply. However, this oxide is not widely used industrially partly because it is not as satisfactory as the other lithium-transition metal oxides, i.e., lithium-cobalt composite oxide (LiCoO2) and lithium-nickel composite oxide (LiNiO2), in charge-discharge characteristics, reducing markedly in cell capacity in the case where the cell is charged and discharged at increasing charge-discharge current values.
In order to prevent deterioration in preservation characteristics, i.e., a reduction in the cell capacity, when the cell is allowed to stand for a prolonged period of time without charging and discharging, and to prevent deterioration in life characteristics, i.e. a diminution in cell capacity, in the case where the cell is repeatedly charged and discharged, studies are underway on lithium ion secondary cells (Japanese Patent No. 3024636) wherein the positive electrode active substance is a mixture of lithium-manganese composite oxide (LiMn2O4) and a lithium-nickel composite oxide [LiNi(1xe2x88x92x)MxO2 wherein 0 less than xxe2x89xa60.5, and M is at least one metal element selected from the group consisting of Co, Mn, Al, Fe, Cu and Sr], and on the partial substitution of an element other than Mn for the Mn in lithium-manganese composite oxide (LiMn2O4).
Nonaqueous electrolyte secondary cells for use in electric vehicles are used under severe conditions involving repetitions of charging and discharging with a great current within a short period of time, and charge-discharge characteristics under such conditions need to be investigated. However, the research on and improvements in lithium ion secondary cells heretofore made are limited almost always to the preservation characteristics and life characteristics as stated above, and exhaustive research still remains to be made on charge-discharge characteristics under conditions involving repetitions of charging and discharging with a great current within a short period of time, i.e., power characteristics. We checked conventional lithium ion secondary cells wherein the positive electrode active substance used is lithium-manganese composite oxide, lithium-nickel composite oxide or a mixture thereof for the evaluation of power characteristics, but were unable to obtain satisfactory results.
An object of the present invention is to give improved power characteristics to lithium ion secondary cells wherein the positive electrode active substance used is a mixture of lithium-nickel-cobalt-manganese composite oxide and lithium-manganese composite oxide.
Accordingly, we have carried out intensive research to fulfill the above object and consequently found that the power characteristics of lithium ion secondary cells are greatly influenced by the composition of lithium-nickel-cobalt-manganese composite oxide, the composition of lithium-manganese composite oxide, the mixing ratio of these two kinds of composite oxides and the average diameter of particles of the two kinds of composite oxides to accomplish the present invention.
The present invention provides a lithium ion secondary cell wherein a positive electrode active substance comprises a mixture of a lithium-nickel-cobalt-manganese composite oxide represented by the formula LiNi(1xe2x88x92xxe2x88x92y)CoxMnyO2 wherein 0.5 less than x+y less than 1.0 and 0.1 less than y less than 0.6 and a lithium-manganese composite oxide represented by the formula Li(1+z)Mn2O4 wherein 0xe2x89xa6zxe2x89xa60.2.
The positive electrode active substance of the lithium ion secondary cell embodying the present invention contains the lithium-nickel-cobalt-manganese composite oxide of the formula LiNi(1xe2x88x92xxe2x88x92y)CoxMnyO2 wherein 0.5 less than x+y less than 1.0 and 0.1 less than y less than 0.6. Presumably, this component gives the active substance a structure permitting lithium ions to be readily inserted into and released from the active substance.
Further by mixing the lithium-nickel-cobalt-manganese composite oxide represented by the formula LiNi(1xe2x88x92xxe2x88x92y)CoxMnyO2 wherein 0.5 less than x+y less than 1.0 and 0.1 less than y less than 0.6 with the lithium-manganese composite oxide represented by the formula Li(1+z)Mn2O4 wherein 0xe2x89xa6zxe2x89xa60.2 and having a spinel structure, the particles of these oxides are held in contact with one another with good stability, presumably resulting in the ease of migration of electric charges in the case where the cell is repeatedly charged and discharged with a great current within a short period of time. It is therefore thought that the lithium ion secondary cell wherein the two kinds of composite oxides are used as the positive electrode substance exhibits outstanding power characteristics.
Stated more specifically, the lithium-nickel-cobalt-manganese composite oxide and the lithium-manganese composite oxide are mixed together in a ratio by weight of 20:80 to 80:20. Presumably, the mixing ratio of the oxides in this range ensures facilitated migration of electric charges between the particles of the composite oxides to result in excellent power characteristics.
Further stated more specifically, the lithium-nickel-cobalt-manganese composite oxide is in the form of particles having an average diameter of 1 to 15 xcexcm, and the lithium-manganese composite oxide is in the form of particles having an average diameter of 5 to 15 xcexcm. Presumably, this ensures ease of migration of charges between the particles of the composite oxides to result in excellent power characteristics.
Thus, the present invention provides a lithium ion secondary cell which is excellent in power characteristics.